The present invention relates to a process for producing a negative-working photosensitive lithographic printing form, in which a negative-working photosensitive mixture which is dissolved in a solvent mixture and which contains a negative-working photosensitive compound and a polymeric binder which is insoluble in water but soluble in aqueous alkaline solution is applied to a base and the layer obtained is subsequently dried.
In the production of lithographic printing forms, the photosensitive mixtures are continuously applied to a base. The coated base is then dried to produce a thin film having uniform thickness. The solvent or solvent mixture should therefore dry on uniformly after coating and exhibit a good surface finish (surface cosmetic), i.e. suppress the occurrence of "fish eyes" precipitates and "clouds". In addition, the solvent or solvent mixture must be capable of evaporating without forming a large residue.
The requirements imposed on a suitable solvent or solvent mixture are limited not only to the immediate period of time which relates to the drying of the mixture, but also to the properties of the solution before application to the base. In this connection, it should be specified in the first place that all the layer constituents are soluble ideally in the necessary concentration. In addition, the solvent or solvent mixture must guarantee a solution stability over a certain period of time-preferably over a few days - without decompositions, precipitates and condensation reactions occurring. In addition, inexpensive availability and as low a toxicity as possible are required.
DE-A 2,034,655 (=US-A 3,790,385) proposes a solvent mixture for a negative-working photosensitive copying material which, according to the examples described therein, is composed of ethylene glycol monomethyl ether, tetrahydrofuran and butyl acetate. That is to say, all of the solvents in the mixture have boiling points below 140.degree. C. It emerged, however, that negative-working photosensitive mixtures which contain such solvent mixtures are less durable, i.e. that precipitates and decompositions appear after only hours. In addition, it is found that the coating and drying properties are only partly advantageous.
DE-A 3,627,585 describes a process for producing presensitized lithographic printing forms based on negative-working photosensitive mixtures in which the photosensitive diazonium resin and the high-molecular-weight binder which is soluble in aqueous alkali can be dissolved in a plurality of different solvents with various boiling points. Mixtures of these solvents are also described.
In particular, mixtures of solvents are described in EP-A 0,184,804 and 0,152,819 (=US-A 4,631,245) (2-methoxyethanol with tetrahydrofuran) and EP-A 0,031,481 (butyl acetate, 2-methoxyethanol and tetrahydrofuran).
DE-A 3,807,406 (=GB-A 2,204,707) discloses a solvent mixture for negative-working photosensitive lithographic plates which comprises 1-methoxy-2-propanol, methyl lactate and methyl ethyl ketone. Up to 2% of a higher-boiling solvent can be added to these low-boiling solvents. With the solvent mixtures described, however, it was not possible to obtain homogeneous solutions. Some of the solids remain behind undissolved.
Seen as a whole, according to the prior art, only very dilute solutions of the layer constituents can be produced, with the result that very large amounts of solvent are required to dissolve all the layer constituents. Layers produced with a solvent mixture also exhibit, on the one hand, layer precipitates, layer defects and cloudy appearance during the drying process. This is associated with a short shelf life of the photosensitive solutions. Frequently, crystallization of layer constituents also occurs, even with a slight reduction in temperature.